I assume that you mean children's meals at Disney?
Yes, they are happy for them to share your food. Just let the waiter know and they will bring another plate etc. Buffets are good as you can get them their own plate of food and not have to pay.
They usually ask the child's age when you make your ADR so the restaurant will always know the child's age when you check in.
We took DD last Spetember when she was just under 3 and had no problems - sometimes she shared our food, sometimes she had her own plate at buffets and sometimes we bought her a kid's meal.